Overview
Understanding the difference between the Decorator and Adapter design patterns is crucial for software designers. These patterns offer solutions for extending or enhancing the functionality of objects (Decorator) and for making unrelated interfaces compatible (Adapter). Recognizing when to use which pattern is essential for creating flexible and maintainable software.
Key Concepts
- Purpose: Decorator adds functionality to objects dynamically, while Adapter makes interfaces compatible.
- Usage scenarios: Use Decorator for extending capabilities without inheritance. Use Adapter to allow communication between incompatible interfaces.
- Implementation: Decorator involves a set of decorator classes that wrap the original class. Adapter involves a separate adapter class that translates calls from one interface to another.
Common Interview Questions
Basic Level
- What is the primary difference between the Decorator and Adapter design patterns?
- Can you explain how the Decorator pattern adds functionality to an object?
Intermediate Level
- How does the Adapter pattern solve interface compatibility issues?
Advanced Level
- Discuss a scenario where combining the Decorator and Adapter patterns would be beneficial.
Detailed Answers
1. What is the primary difference between the Decorator and Adapter design patterns?
Answer: The primary difference lies in their purposes and applications. The Decorator pattern is used to add new functionality to an object dynamically, without altering its structure. This is achieved by creating a set of decorator classes that wrap the original class. In contrast, the Adapter pattern is used to make two incompatible interfaces compatible, enabling objects with incompatible interfaces to communicate. It does so by implementing an adapter class that translates or adapts calls from one interface to the other.
Key Points:
- Decorator enhances objects at runtime.
- Adapter makes incompatible interfaces work together.
- Decorator uses composition, Adapter can use composition or inheritance.
Example:
// Decorator example in C#
public interface IComponent
{
string Operation();
}
public class ConcreteComponent : IComponent
{
public string Operation()
{
return "ConcreteComponent";
}
}
public class Decorator : IComponent
{
private readonly IComponent _component;
public Decorator(IComponent component)
{
_component = component;
}
public virtual string Operation()
{
return _component.Operation();
}
}
public class ConcreteDecoratorA : Decorator
{
public ConcreteDecoratorA(IComponent component) : base(component) {}
public override string Operation()
{
return $"ConcreteDecoratorA({base.Operation()})";
}
}
2. Can you explain how the Decorator pattern adds functionality to an object?
Answer: The Decorator pattern adds functionality to an object by wrapping it with a decorator class. This class implements the same interface as the object and holds a reference to it. By doing this, it can execute its own behavior before or after delegating the call to the wrapped object. Decorators can be stacked, meaning an object can be wrapped in multiple decorators, each adding its own behavior.
Key Points:
- Decorators wrap objects to add functionality.
- They implement the same interface as the object they wrap.
- Multiple decorators can be used to add multiple functionalities.
Example:
// Continuing the previous Decorator example with ConcreteDecoratorB
public class ConcreteDecoratorB : Decorator
{
public ConcreteDecoratorB(IComponent component) : base(component) {}
public override string Operation()
{
return $"ConcreteDecoratorB({base.Operation()})";
}
}
// Usage
IComponent component = new ConcreteComponent();
component = new ConcreteDecoratorA(component);
component = new ConcreteDecoratorB(component);
Console.WriteLine(component.Operation());
// Output: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))
3. How does the Adapter pattern solve interface compatibility issues?
Answer: The Adapter pattern solves interface compatibility issues by acting as a bridge between two incompatible interfaces. It does this through an adapter class that translates, or adapts, the interface of one class to the interface another class expects. This allows classes with incompatible interfaces to work together without changing their existing code.
Key Points:
- Adapter acts as a bridge between two incompatible interfaces.
- It translates calls from one interface to another.
- Enables collaboration without altering existing code.
Example:
// Adapter pattern in C#
public interface ITarget
{
string GetRequest();
}
public class Adaptee
{
public string GetSpecificRequest()
{
return "Specific request.";
}
}
public class Adapter : ITarget
{
private readonly Adaptee _adaptee;
public Adapter(Adaptee adaptee)
{
_adaptee = adaptee;
}
public string GetRequest()
{
return $"This is '{_adaptee.GetSpecificRequest()}'";
}
}
// Usage
Adaptee adaptee = new Adaptee();
ITarget target = new Adapter(adaptee);
Console.WriteLine(target.GetRequest());
// Output: This is 'Specific request.'
4. Discuss a scenario where combining the Decorator and Adapter patterns would be beneficial.
Answer: A scenario where combining the Decorator and Adapter patterns would be beneficial is when working with a third-party library or system with a set of given interfaces, and you need to not only adapt these interfaces to work with your system but also to extend their functionality. By first adapting the third-party interface to fit your system's expectations (using the Adapter pattern) and then wrapping the adapted class with one or more decorators (using the Decorator pattern), you can achieve both compatibility and extended functionality without altering the original codebase.
Key Points:
- Combining patterns for compatibility and extended functionality.
- Adapter first to ensure interface compatibility.
- Decorator next to add additional functionalities.
Example:
// Assuming the Adaptee and Adapter from the previous example, we add a decorator
public class EnhancedAdapter : Decorator
{
public EnhancedAdapter(ITarget target) : base(target) {}
public override string Operation()
{
return $"Enhanced {base.Operation()}";
}
}
// Usage
Adaptee adaptee = new Adaptee();
ITarget target = new Adapter(adaptee);
target = new EnhancedAdapter(target);
Console.WriteLine(target.GetRequest());
// Output: Enhanced This is 'Specific request.'
This example demonstrates how an Adapter can be used to make an incompatible interface compatible, and a Decorator can then add extra functionality to the adapted class.
